Ionic mechanisms of two types of on-center bipolar cells in the carp retina. I. The responses to central illumination.

Abstract

Properties of the depolarizing response of on-center bipolar cells to a light spot stimulus were studied in the carp [Cyprinus carpio] retina. On-center bipolar cells were classified into 2 types, cone-dominant and rod-dominant, according to their major input from cones and rods. Cone-dominant bipolar cells responded to spectral light with the maximum amplitude near 625 nm, suggesting major input from red cones. The response was accompanied by a resistance increase and showed a reversal potential at -63 .+-. 21 mV when the membrane was hyperpolarized by current. The photoresponse of cone-dominant cells is probably due to a decrease of membrane conductances to K (gK) and Cl (gCl). Rod-dominant bipolar cells responded to spectral light with the maximum amplitude near 525 nm under scotopic conditions and near 625 nm under photopic conditions, providing evidence that they receive input from rods and red cones. In the scoptopic condition their response was accompanied by a resistance decrease and showed a reversal potential at 29 .+-. 13 mV, whereas in the photopic condition the response in most of them was accompanied by a resistance increase, at least in their initial part and showed a reversal at -53 .+-. 11 mV. The photoresponse activated by rod input is probably due to an increase in gNa. In the mesopic condition rod-dominant cells showed complex electrical membrane properties as the result of electric interaction between the above 2 different ionic mechanisms activated by rod and cone inputs.